1. Field of Invention
This invention pertains to telecommunications, and particularly to the handling of cells in a switching node of a telecommunications network operating in the asynchronous transfer mode.
2. Related Art and Other Considerations
The increasing interest for high band services such as multimedia applications, video on demand, video telephone, and teleconferencing has motivated development of the Broadband Integrated Service Digital Network (B-ISDN). B-ISDN is based on a technology know as Asynchronous Transfer Mode (ATM), and offers considerable extension of telecommunications capabilities.
ATM is a packet-oriented transfer mode which uses asynchronous time division multiplexing techniques. Packets are called cells and have a fixed size. An ATM cell consists of 53 octets, five of which form a header and forty eight of which constitute a "payload" or information portion of the cell. The header of the ATM cell includes two quantities which are used to identify a connection in an ATM network over which the cell is to travel, particularly the VPI (Virtual Path Identifier) and VCI (Virtual Channel Identifier). In general, the virtual is a principal path defined between two switching nodes of the network; the virtual channel is one specific connection on the respective principal path.
At its termination points, an ATM network is connected to terminal equipment, e.g., ATM network users. Between ATM network termination points are a plurality of switching nodes having ports which are connected together by physical transmission paths or links. In traveling from an origin terminal equipment to a destination terminal equipment, ATM cells forming a message may travel through several switching nodes.
A switching node has a plurality of ports, each of which is connected by via a link circuit and a link to another node. The link circuit performs packaging of the cells according to the particular protocol in use on the link. A cell incoming to a switching node may enter the switching node at a first port and exit from a second port via a link circuit onto a link connected to another node. Each link can carry cells for a plurality of connections, a connection being a transmission between a calling subscriber or party and a called subscriber or party.
The switching nodes each typically have several functional parts, a primary of which is a switch core. The switch core essentially functions like a cross-connect between ports of the switch. Paths internal to the switch core are selectively controlled so that particular ports of the switch are connected together to allow a message ultimately to travel from an ingress side of the switch to an egress side of the switch, and ultimately from the originating terminal equipment to the destination terminal equipment.
FIG. 1 shows a prior art ATM switching device 10 having a switch core 12, a plurality of ingress ports 14 (1)-14 (3), and a exemplary egress port 16. FIG. 1 shows, in broken lines, cells incoming to switch 10 on three channels, particularly a first channel VC-1 to port 14 (1), a second channel VC-2 to port 14 (2), and a third channel VC-3 to port VC-3. All cells on the same channel have the same VC or VCI, e.g., cells on the first channel all have a first VC denominated as VC-1.
In the illustration of FIG. 1, the incoming cells on all channels VC-1 through VC-3 happen to be switched through switch core 12 to the same egress port 16 for eventual output to channel VC-4. At egress port 16 the cells are all assigned a new VPI/VCI code. The switching of several incoming VC channels to a same outgoing VC channel is referred to as "VC merging". The purpose of VC merging is to collect packets that shall be routed to the same destination into one single virtual channel, instead of using a separate VC between each source and the destination. This reduces the number of required virtual channels in the ATM network.
If the cells were all sent out in the order in which they arrived at the switch core 12, there would be no way of separating them at the end-point of channel VC-4. For this reason, VC merging can only work for channels carrying AAL-5 packets by storing complete packets at the egress port and sending them out packet-by-packet, i.e., the multiplexing is performed at the packet level.
What is needed, therefore, and an object of the present invention, is method and apparatus for efficiently conducting a VC merging operation.